In various industrial contexts, it is commonly required to provide structural bearings for supporting vertical loads while preventing transfer of significant lateral forces to the supporting structure. Examples include structural bearings in bridges and larger buildings that must be able to carry large vertical loads without allowing transfer of lateral loads to the supporting structure due to wind loads, seismic loads, or expansion or contraction induced by temperature changes. As well, it is commonly desirable to prevent the development of lateral reactions against supporting structures and foundations that can otherwise develop in some structures due to inherent structural characteristics. For example, ‘rigid frame’ building structures can in some cases exert lateral forces against supporting structures or foundations, even under vertical loading alone.
In such situations, prevention of lateral load transfer to the supporting structure, or prevention of lateral reactions in rigid frame structures, is commonly achieved by allowing the bearings to move laterally relative to the supporting structure, with such lateral movement being facilitated by rollers of some type, or the bearings may be slide bearings using a low-friction material such as PTFE (polytetrafluoroethylene).
In other scenarios, it is necessary to temporarily support large vertical loads on a supporting structure without transferring lateral loads, such as in conjunction with cantilevered mobile drilling rigs used to drill closely-spaced oil wells, particularly in extremely cold conditions. In such drilling operations, multiple wells are drilled at linear spacings of 10 or 12 feet, with the wellheads being disposed within a heated enclosure. The roof of the wellhead enclosure has hatches spaced to match the well spacing. Well drilling is carried out using a wheel-mounted or track-mounted mobile drilling rig having a cantilevered superstructure that carries a typically sliding rig floor. The mobile rig is positioned adjacent to the wellhead enclosure with the cantilevered superstructure extending over and beyond the wellhead enclosure. The mobile rig is movable parallel to the line of wells, such that it can be longitudinally aligned with each well location as required.
When the mobile rig is longitudinally aligned with a selected well, the free end of the cantilevered superstructure must be supported before the rig floor and mast section can be laterally positioned over the well and drilling operations commenced. For this purpose, a heavy girder is installed adjacent to the wellhead enclosure on the side opposite the mobile rig. The cantilevered superstructure is provided with two or more telescoping support legs that can be extended to bear upon the girder, without any mechanical connection or anchorage to the girder. When it is desired to move to a new well location, the support legs are retracted vertically away from the girder, and the mobile rig can then be relocated as required.
The girder is typically supported by spaced columns such that the top of the girder is at an elevation well above the roof of the wellhead enclosure, which may put the girder 25 or 30 feet above the ground. The vertical load exerted on the girder by each support leg during well-drilling operations can be in the range of 500,000 to 600,000 pounds. These large vertical loads create high frictional resistance across the contact interface between the support legs and the girder, such that large lateral loads exerted on the drilling rig structure by wind or seismic forces will react in part against the girder. This is undesirable not only because of the resultant torsional stresses induced in the girder, but also because of the resultant large bending moments induced in the structural columns supporting the girder (not to mention lateral loads and bending moments induced in the piles or other foundation systems supporting the columns).
For the foregoing reasons, there is a need for improved structural support bearings that will transfer large vertical loads to a supporting structure without allowing the transfer of significant lateral loads the supporting structure, but also without requiring lateral displacement at the contact interfaces between the support bearings and the supporting structure. The present invention is directed to this need.